In general, as tea is fermented by microorganisms, its taste, scent and color are changed due to the metabolism by the microorganisms, leading to change in flavor. That is to say, as the carbohydrates, proteins, catechins and minerals of the tea are metabolized by the characteristic enzymes of the microorganisms, the composition of organic acids, polyphenols, etc. of the tea is changed, thereby resulting in change of taste, scent and color. The change of the flavor of the tea upon fermentation depends on the related microorganism, contents of the tea ingredients, composition of the protein-free fermentation broth, fermentation temperature, fermentation time, or the like.
Thus, in order to produce good fermented tea, selection of adequate microorganisms, composition of the protein-free fermentation broth, determination of the fermentation temperature and time, or the like are important. In general, fermented tea is obtained by leaving heat-treated tea leaves alone in a container. Thus, the fermentation is affected by airborne microbes or unspecified anaerobes or other microbes, especially fungi, attached to the tea leaves. Specifically, toxins produced by pathogenic microorganisms such as Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, etc. and toxins produced by fungi, e.g. aflatoxin, may be hazardous. Furthermore, it is difficult to provide tea with reproducible taste because the kind and quantity of the microorganisms cannot be controlled.